• Four patients who entered silos within a day or two after filling with corn silage noted irritating fumes and experienced respiratory symptoms of varying degrees of severity. A period of failure to improve was followed by a period of worsening that ended with death on the 27th and 30th days in two cases. The disease was halted in the other two cases by treatment with prednisone. Intramuscular injections of this drug in 10 mg. doses three times daily were followed by prompt and striking improvement, especially in one case. Chemical evidence showed that the toxic gas was probably nitrogen dioxide, a gas capable of causing early pulmonary edema with subsequent bronchiolitis fibrosa obliterans. The bronchiolitis gives a roentgenogram marked by innumerable discrete nodular densities in the lungs and indistinguishable from that of acute miliary tuberculosis; its nature was verified at autopsy in the two fatal cases. Oxygen gave only symptomatic improvement, and the effects of bronchodilators and antibiotics were only temporary. Prevention of silo-filler's disease depends on adequate ventilation, on means of keeping people and animals from straying into dangerous spaces adjoining a silo, especially during the 7 to 10 days after the silo is filled, and on a wider recognition of the danger.